1. Field of the Invention
The present invention generally relates to a digital mobile communication system. More specifically, the present invention is directed to a technique for mutually synchronizing radio communication channels among a plurality of radio base stations, through which mobile terminals transmit/receive signals to/from these plural radio base stations.
2. Description of the Prior Art
FIG. 1 schematically illustrates a conceptional diagram of a service area covered by one generic mobile communication system. In this conceptional diagram, symbols BS1 to BS4 indicate radio base stations, and these radio base stations constitute radio communication areas 101 to 104, respectively. When a so-called "hand over" operation is carried out while a mobile terminal under communication is moved from a certain radio communication area to another radio communication area, a problem occurs.
FIG. 2 shows the principle of the problem which will be described below.
Assume now that a mobile terminal is using a time slot (T2) of a radio communication channel through which this mobile terminal is being communicated with a radio base station (BS1) is synchronism with this mobile terminal, and communication channels of BS2 are not synchronized with BS1 but those of BS3 are synchronized with BS1. When this mobile terminal tries to use another time slot (T2' or T2") of another radio communication channel served by another radio base station (BS2 or BS3), this mobile terminal must newly establish the synchronization between BS2 and the own mobile terminal if the hand-over operation is carried out from BS1 to BS2, although such reestablishment of the synchronization is not needed for the hand-over operation from BS1 to BS3. Due to this resynchronizing operation, there is a problem that telephone communications may be interrupted.
As the synchronizing technique among the radio base stations required in the digital mobile communication system, there are disclosed the technique for mutually monitoring radio communication channels among radio base stations to establish synchronization, the technique for receiving a reference clock emitted from a GPS (global positioning system) satellite to establish synchronization, and the technique for establishing synchronization in response to a signal supplied from a wired communication network (telephone network) connected to a mobile communication system. Among these synchronizing techniques, the technique for mutually monitoring the radio communication channels among the radio base stations may have the following difficulties. That is, the signal must be received by employing the transmission slot, and since the C/N (receive carrier-to-noise ratio) is not sufficient, depending upon the distance between the own radio base station and the counter radio base station, erroneous synchronization will occur. Also, the synchronizing to technique with employment of the GPS satellite owns such a problem that since the receiver apparatus capable of receiving the GPS signal must be assembled into all of the radio base stations, these radio base stations require large-scale and expensive facilities. Furthermore, this GPS type synchronizing technique owns another problem that the GPS electromagnetic waves may not be received, depending on the installation places of the radio base stations, such as places located behind buildings along the GPS signal receiving direction.
The above-described synchronizing technique in response to the signal supplied from the wired communication network is described in, for instance, Japanese Laid-open Patent Application No. 2-238732, and No. 3-224325.
In accordance with the synchronizing technique described in Japanese Laid-open Patent Application No. 2-238732, the timing information is sent out as the reference from the switching unit for constituting the mobile switching network to each of the radio base stations. In the respective radio base stations, when one mobile terminal under communication with another radio base station issues a demand to switching the communication from this radio base station to the own radio base station, the up-link radio signal of this mobile terminal which is now communicated with another radio base station is monitored, and the timing for constituting the reference, supplied from the switching unit, is newly matched with the timing for communication purpose in response to the monitored timing.
As indicated in FIG. 3, the synchronizing technique described in Japanese Laid-open Patent Application No. 3-224325 is realized by arranging the synchronization signal generating apparatus 6 in combination with the control station 1. Then, the radio base stations (BS) 3-a and 3-b are provided under control of the control station 1, and the delay amount adjusting apparatuses 7-a and 7-b are arranged in combination with these radio base stations (BS). The synchronization is established between the base stations since the reset pulses sent from the synchronization signal generating apparatus 6 reaches all of the base stations under control of the control station 1 at the same time. To this end, the transmission delays depending on the transmission paths from the control station 1 to the respective base stations are measured in advance, and the adjustment values are set to the delay adjusting apparatuses 7-a and 7-b provided in combination with the respective base stations so as to make the delay amounts to the respective base stations constant. As a consequence, the reset pulses can be simultaneously supplied to all of the base stations.
The above-described conventional synchronizing technique described in Japanese Laid-open Patent Application No. 2-238732 owns the following problems. That is, the radio base station has to monitor the radio communication channel used by the mobile terminal, which is currently communicating with another radio base station and will perform the hand over operation, for establishing the synchronization by the reference timing received from the switching unit. Every time the synchronization is established, the radio base station requires the monitoring control, resulting in increasing of the load given to the radio base station. Moreover, although the synchronization can be established between the own radio base station and the counter radio base station monitored by the mobile terminal, the synchronization cannot be secured between the own radio base station and other adjoining radio base stations.
Also, the above-mentioned conventional synchronizing technique disclosed in Japanese Laid-open Patent Application No. 3-224325 has the following drawbacks. That is, the delay amounts are previously measured which are set to the delay adjusting apparatuses employed in the respective radio base stations, and these measured delay amounts are set by making telephone instructions to the respective radio base stations. Under such circumstances, in a large-scale mobile communication network, a lengthy delay-amount measuring time is necessarily required in order that the reset pulses should reach all of the radio base stations at the same time, and further the cumbersome delay-amount setting sequential operations are required.
In addition, any of the above-described conventional synchronizing techniques are directed to the synchronization establishment among the radio base stations under control of the mobile communication controlling switching unit provided within the own mobile communication network, but not directed to the synchronization establishment between the radio base station of the own mobile communication network and another radio base station of a mobile communication network different therefrom. As a consequence, even when a plurality of mobile communication networks managed by plural mobile communication utilities are provided in adjacent areas and these mobile communication utilities have allowed roaming, if one mobile terminal tries to use another mobile communication network of the different mobile communication utility, then the synchronization must be reestablished. Thus, a telephone communication is instantaneously interrupted when the mobile communication networks are switched.